System and method for reducing peak-to-average power ratio for multi-carrier communication systems

ABSTRACT

The present disclosure presents a predictive signal producing method that effectively levels transmitter output power in a multi-carrier communication system and results in approaching amplifier performance normally associated with constant carrier waveforms. Embodiments of the disclosed solution offers &gt;10 dB reduction in the peak-to-average power required to support the transmission of, for example, orthogonal frequency division modulation (“OFDM”) modulation techniques. Embodiments of the novel system and method maximize peak-to-average power ratio (“PAPR”) reduction with selective mapping and soft clipping, which may include filtering, combined. This novel approach also minimizes overhead, bit error rate, retransmissions, and increases latency as well as implementing processing cycles with a number of iterations. The disclosed system and method improves the total system DC power efficiency and provides an optimal solution for PAPR reduction in multi-carrier communication systems such as, for example, OFDM.

RELATED APPLICATIONS

This application is a divisional of and claims priority of U.S. patentapplication Ser. No. 10/690,613 entitled “System and Method for ReducingPeak-to-Average Power Ratio for Multicarrier Communication Systems”filed on Oct. 23, 2003 now U.S. Pat. No. 7,639,747, the entirety ofwhich is herein incorporated by reference.

BACKGROUND

The peak-to-average power ratio (“PAPR”), also known as peak-to-meanpower ratio (“PMPR”) or peak factor, is an important characteristic ofmulti-carrier transmitted signals. The peak of the signal can often be Ntimes greater than the average power level. These large peaks causeintermodulation distortion which can result in an increase in the errorrate. These distortions are brought about from the limitations inherentin a transmitting amplifier.

In order to prevent the transmitter amplifier from limiting (clipping),the average signal power must be kept low enough to keep the signalrelatively linear through the amplifier. In order to transmit a highpower signal, a high power amplifier is required which requires a largeDC system power. A much higher power amplifier is required to transmitmulti-carrier waveforms than for constant envelope waveforms. Forexample, using 64 carrier waveforms, a 40 dBm power amplifier wouldrequire about 15 dB of back off. Therefore, instead of operation at 40dBm (10 watts) the amplifier is only capable of operating at 25 dBm(0.316 Watts). Thus in order to transmit at the desired 40 dBm, a 55 dBm(316 Watt) amplifier would be required. The associated power supply,power consumption, can be substantially increased. In addition, suchlarge power requirements lead to associated increased space demands andheat dissipation requirements.

With the large amount of interest and activity with Orthogonal FrequencyDivision Modulation (“OFDM”), and in particular 802.11a and 802.11gcommunication technology, the PAPR problem is exaggerated, 802.11 withits use of complex waveforms requires highly linear RF amplifiers.Current 802.11 physical layer integrated circuits have not implementedPAPR reduction schemes. In particular, multi-tone OFDM typicallyrequires greater than 10 dB power amplifier back-off because of a highpeak-to-average power ratio. The net result of these factors is anincreased DC power demand beyond that encountered with other 802.11techniques. The effect may be less noticeable for short duty cyclesignals, but can be significant for situations requiring continuoustransmission of data.

OFDM, as mentioned above, is a method of transmitting datasimultaneously over multiple equally-spaced and phase synchronizedcarrier frequencies, using Fourier transform processing for modulationand demodulation. The method has been proposed and adopted for manytypes of radio systems such as wireless Local Area Networks (“LAN”) anddigital audio and digital video broadcasting. OFDM offers manywell-documented advantages for multi-carrier transmission at high datarates, particularly in mobile applications. Specifically, it hasinherent resistance to dispersion in the propagation channel.Furthermore, when coding is added it is possible to exploit frequencydiversity in frequency selective fading channels to obtain excellentperformance under low signal-to-noise conditions. For these reasons,OFDM is often preferable to constant envelope modulation with adaptiveequalization and is arguably less complex to implement.

The principal difficulty with OFDM, as alluded to above, is that whenthe sinusoidal signal of the N carriers add mostly constructively, thepeak envelope power is as much as N times the mean envelope power. Ifthe peak envelope power is subject to a design or regulatory limit thenthis has the effect of reducing the mean envelope power allowed underOFDM relative to that allowed under constant envelope modulation. Ifbattery power is a constraint, as is typically the case with portableequipment such as mobile consumer appliances, laptops, and sophisticatedDepartment of Defense communication systems, then the power amplifiersrequired to behave linearly up to the peak envelope power must beoperated inefficiently with considerable back-off from compression.Digital hard limiting of the transmitted signal has been shown toalleviate the problem but only at the cost of spectral sidelobe growthand consequential bit error performance degradation.

FIG. 1 illustrates 16 carriers in-phase with frequencies ranging from Chertz for carrier 101 to 16 C hertz for carrier 116, with theintermediate carriers having increasing frequencies with steps of C,characteristic of an OFDM signal. Each of the carriers as shown have anominal maximum amplitude of one, however as seen in FIG. 2 thedisparate effects of the carriers added in-phase are readily apparent.FIG. 2 shows the large peak amplitudes of the added carriers at aroundtime sample 25 and around time sample 1575.

FIG. 3 illustrates the peak-to-average power ratio of the 16 carriersmodulated in-phase. The large peak-to-average power ratios correspond tothe large amplitude spikes illustrated in FIG. 2. The peak-to-averagepower ratio for FIG. 3 is generated according to the function:

$\frac{P_{peak}}{P_{avg}} = {X_{i}^{2}/\left( {\frac{1}{N}{\sum\limits_{1}^{N}X_{i}^{2}}} \right)^{2}}$where X_(i) is the signal sample amplitude at sample number i and N isthe number of samples of the multi-carrier symbol.

These problems provide a clear motivation to find other solutions forcontrolling the peak to mean envelope power ratio of the transmittedsignal. One solution offered uses block coding to transmit across thecarriers only those poly-phase sequences with small PAPR; however, thisentails an exhaustive search to identify the best sequences and requireslarge look-up tables for encoding and decoding.

Some techniques, such as spectral whitening, serve to reduce thepeak-to-average ratio and allow the use of RF amplifiers closer to their1 dB compression point, resulting in a decreased DC power demand. Someprior art solutions have used clipping or mapping to reduce the PAPR.However no solutions have employed or suggested a hybrid system,including selective mapping and soft clipping as is presented in thisdisclosure.

It is an object of the present disclosure to obviate the disadvantagesof the prior art and present a novel system and method for reducing thepeak-to-average power ratio of a signal for transmission in amulti-carrier communication system. One method sequences informationdata according to a data vector and modulates multi-carrier symbols withthe sequenced data. The resultant modulated data signal'speak-to-average power ratio is measured and compared to a predeterminedthreshold. In the method, if the peak-to-average power ratio exceeds thepredetermined threshold, the data is re-sequenced in accordance with anew data vector and repeats the modulation and comparison processes.Otherwise the modulated data signal is appended with a data mapassociated with the respective data vector and sampled. Those modulateddata signal samples which exceed a predetermined range are clipped andthe clipped modulated data signal is filtered, thereby reducing the PAPRratio of the signal to be transmitted in a multi-carrier communicationsystem.

It is a further object of the present disclosure to present a novelsystem and method, in a multi-carrier communication system, oftransmitting data. An embodiment of a system and method includessequencing the data according to one or more unique sequences,modulating one or more of the sequences of data and selecting one of themodulated sequences of data, based on the PAPR. The system and methodfurther include filtering the selected modulated sequence of data toremove amplitude peaks outside a threshold band, and transmitting thefiltered signal over the multi-carrier communication system.

It is another object of the present disclosure to present in amulti-carrier communication system with a linear amplifier, a novelsystem and method of preventing limiting of the amplifier. The novelsystem and method include sequencing data to be transmitted based uponthe resultant PAPR from the modulation of the sequenced data. Alsoincluded is sampling the modulated sequenced data and truncating thesamples which are outside a threshold, thereby forming a data signalthat prevents limiting of the amplifier.

It is still another object of the present disclosure to present, in amulti-carrier communication system for transmitting data, a novel systemand method for forming a data signal that reduces the required power ofa transmitter. The novel system and method includes providing the datato be transmitted in one or more unique sequences and modulating the oneor more unique sequences thereby creating one or more unique modulatedsequences. The system and method may also include selecting fortransmission one of the unique modulated sequences based on itsassociated PAPR, and truncating amplitudes of the selected sequencewhich are outside a predetermined range to thereby form a data signalthat reduces power required to transmit the signal.

It is yet another object of the present disclosure to present a noveltransmitter for transmitting data with multiple carriers. A transmittermay have a modulator for modulating multi-carrier symbols with the data,a processor for measuring the PAPR of the modulated data, and a logicdevice for comparing the PAPR with a threshold. The transmitter may alsohave a processor for deterministically re-sequencing the data and anamplitude filter for reducing peaks of the modulated data signal thatare outside a predetermined range.

The present disclosure presents a predictive signal producing methodthat effectively levels transmitter output power, and results inapproaching amplifier performance normally associated with constantcarrier waveforms of the prior art. This solution offers >10 dBreduction in the peak-to-average power required to support thetransmission of OFDM modulation techniques. This approach maximizes PAPRreduction with selective mapping and soft clipping combined. Theapproach also minimizes overhead, bit error rate, retransmissions, andincreases latency as well as implementing processing cycles with anumber of iterations. The disclosed approach improves the total systemDC power efficiency and provides an optimal solution for PAPR reductionin OFDM and is uniquely different from the prior art.

These and many other objects and advantages of the present disclosurewill be readily apparent to one skilled in the art to which thedisclosure pertains from a perusal of the claims, the appended drawings,and the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of N=16 in-phase carriers typical of an OFDMsystem.

FIG. 2 is a representation of the amplitude vs. time of the carriers ofFIG. 1 added in-phase.

FIG. 3 is a representation of the PAPR of the added carriers vs. time.

FIG. 4 is an overview of a PAPR reduction system and method according todisclosed embodiments.

FIG. 5 is an overview of a soft-clipping method according to disclosedembodiments.

FIG. 6 is a graphical representation of a windowing operation accordingto disclosed embodiments.

FIG. 7 is an overview of a hardware implementation of disclosedembodiments.

DETAILED DESCRIPTION

An overview of a disclosed embodiment for a method of reducing PAPR isshown in FIG. 4. The data to be transmitted is gathered in block 401.This data contains the information to be transferred from a data source.The data source being a computer, laptop, mobile phone, or appliance orother data generation, relay, input or storage source. The sequence ofthe data is described in a data vector. In modulator 402, amulti-carrier symbol is modulated with the assembled data. Themulti-carrier system preferably being OFDM or other 802.11 multi-carriersystems, however embodiments of other non-802.11 multi-carrier systemsare also envisioned.

The resultant PAPR of the modulated carriers containing the sequenceddata, which is described with a data vector, is measured in block 403 a.The measured PAPR is then compared with a predetermined threshold inblock 403 b. The threshold may be, for example, user determined orderived from a regulatory requirement. It is also envisioned that thethresholds may be selected from a look up table or other empiricalmethod. If the measured PAPR of the data vector is above the appropriatethreshold, meaning the PAPR is too high, the data is deterministicallyscrambled (or re-sequenced) in block 404 to create a different datavector, of course the different data vector contains the same data, justwith a different or unique sequence. The new scrambled data or datavector is modulated with the multi-carrier symbols in block 402. ThePAPR of the new iterative data vector is then again measured. If thePAPR is again above the threshold the data is again deterministicallyscrambled to create yet another different data vector and continues theprocess through modulation, measurement and scrambling until the PAPR ofthe respective data vector is within the threshold. This process is notdirected towards obtaining the best sequence but rather directed toobtaining an acceptable sequence. However, if the PAPR is less than thethreshold then a data mapping byte(s) descriptive of the data vector,(indicating the scrambled sequence of data) is appended to the modulateddata in block 405. The data mapping byte is used to descramble the dataon the receiver end. The modulated scrambled data and appended mappingbyte is then passed to block 406 which passes the symbols to a softclipping algorithm.

With reference now directed to FIG. 5, the entire packet of themodulated data and appended mapping byte is sampled or processed frommemory in block 501. The samples X_(i) are compared to a secondthreshold as indicated in block 502. If the sample X_(i) exceeds thethreshold, a windowing operation or filtering operation is applied tothe packet in which the samples which exceed the threshold are reducedor clipped. In addition to reducing X_(i), it is also advantageous toreduce adjacent samples {X_((i−K)) . . . X_((i−2)), X_((i−1)),X_((i+1)), X_((i+2)) . . . X_((i+K))}, where K can be experimentally,theoretically or empirically determined as shown in block 503. Thefiltering could be a Gaussian-shaped filter, which has the samefrequency and time-domain characteristics. The filtering operation, forexample, can be implemented as a FIR or IIR, such as h(n)={0.75, 0.5,0.75}. The filtering operation can also be adaptive using a measuredparameter of the filter output to adapt the filter operation.

FIG. 6 shows a representative example of the soft clipping algorithm.The second thresholds are shown as the window borders 601 a and 601 b.Those amplitudes that are above threshold 601 a and/or below threshold601 b, labeled collectively as 600, are clipped or truncated by methodsknown in the art. Thus the soft clipping algorithm filters out peaksabove a dynamic threshold.

The result of the soft clipping process may increase out of band noise;however this effect can be ameliorated by a raised cosine filter orother appropriate filter. The second or windowing threshold may bedynamically adjusted based on several factors such as, but not limitedto, the measured PAPR of the accepted data vector or may bepredetermined by the user. The use of look-up tables for establishingthe second threshold is likewise envisioned.

The PAPR method described above is envisioned to be implemented viaeither hardware or software, or both. A hardware implementation of anembodiment of a PAPR reduction system is shown in FIG. 7. In FIG. 7 the802.11a MAC layer 701 is connected by a data bus 702 a to a digitalsignal processor 703. The DSP is connected to the digital to analogconverter 704 and the 802.11a physical layer 705 by respective databusses 702 b and 702 c. The physical layer 705 supplied with a basebandor intermediate frequency IF signal to an analog-to-digital (“A/D”)Converter 706 which connects back to the DSP by data bus 702 d.

The output of the digital to analog Converter 704 supplies an IF signalto the up-converter 709 which generates a radio frequency RF signal, orsignal at a frequency applicable to the transmission media, to the powerAmplifier 707 which amplifies the signal for transmission over thetransmission channel by antenna 708 or by a transducer applicable to thetransmission media.

The present disclosure presents a predictive signal-producing algorithmthat effectively levels transmitter output power, and results inapproaching amplifier performance normally associated with constantcarrier waveforms of the prior art. The present disclosure also presentsa technique to reduce the PAPR of waveforms produced from amulti-carrier chipset, preferably OFDM or other multi-carrier waveformproducing chipsets, by using external methods to the chipset. Prior artrequires modification to the actual waveform producing chipset (which istypically not possible) in order to implement PAPR reduction techniques.The technique presented here is unique in that it provides a method toreduce PAPR without modification to the chipset.

While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and that the scope of the invention is to be definedsolely by the appended claims when accorded a full range of equivalence,many variations and modifications naturally occurring to those of skillin the art from a perusal hereof.

1. In a multi-carrier communication system, a method of transmittingdata comprising the steps of: (a) sequencing the data according to oneor more unique sequences; (b) modulating one or more of the uniquesequences of data to create a modulated signal; (c) selecting one of themodulated sequences of data based on a comparison of the peak-to-averagepower ratio (PAPR) of that sequence to a first threshold value; (d)appending a data map signal associated with the selected sequence tocreate an appended signal; (e) filtering said appended signal to removeamplitude peaks outside a second threshold band to thereby create afiltered signal; and, (f) transmitting the filtered signal over themulti carrier communication system.
 2. The method according to claim 1,wherein the step of filtering includes the step of comparing samples ofthe selected one to a threshold and reducing the amplitude of samplesexceeding the threshold.
 3. The method according to claim 2, furthercomprising the step of reducing the amplitude of samples adjacent to thesamples exceeding the threshold.
 4. In a multi-carrier communicationsystem with a linear amplifier, a method of preventing limiting of theamplifier comprising the steps of: (a) sequencing data to be transmittedbased upon a resultant comparison of the peak-to-average power ratio(PAPR) of the modulated sequence to a first threshold value; (b)modulating the sequenced data; (c) appending a data map signalassociated with the sequenced data to create an appended signal; (d)sampling the appended signal; (e) reducing the amplitude of sampleswhich are outside a predetermined threshold; and, (f) transmitting theresultant signal with a reduced PAPR to thereby prevent limiting of theamplifier.
 5. The method according to claim 4, further comprising thestep of reducing the amplitude of samples adjacent to the samplesoutside a predetermined threshold.
 6. In a multi-carrier communicationsystem for transmitting data, a method for forming a data signal thatreduces the required power of a transmitter comprising the steps of: (a)providing the data to be transmitted in one or more unique sequences;(b) modulating the one or more unique sequences thereby creating one ormore unique modulated sequences; (c) selecting for transmission one ofthe one or more unique modulated sequences based on a comparison of thepeak-to-average power ratio of the unique modulated sequences to a firstthreshold value; (d) appending a data map signal associated with theselected sequence to create an appended signal; and (e) reducingamplitudes of the appended signal which are outside a predeterminedrange to thereby form a data signal that reduces power required totransmit the signal.
 7. The method according to claim 6, wherein thestep of reducing amplitudes includes the step of comparing samples ofthe selected one to a threshold and reducing the amplitude of samplesexceeding the threshold.
 8. The method according to claim 7, furthercomprising the step of reducing the amplitude of samples adjacent to thesamples exceeding the threshold.